This invention relates to a medium for recording by means of an ink and, more particularly, to an ink-jet recording medium excellent in the density of recorded images and characters, in the ink absorbency, and in the durability of recorded images.
The ink-jet recording system provides the recording of images and characters by causing tiny ink droplets to fly on various working principles and to adhere to a recording medium such as paper. This system is coming in wide use as a means for recording a variety of patterns including "kanji" (chinese-derived characters) and color images because of high speed, low noise, simplicity in multicolor recording, versatility of recorded patterns, and, in addition, no need of development nor fixing. It is also possible by the multicolor ink-jet system to obtain a recorded image comparable favorably to that obtained by multicolor printing using a printing plate or by color photography. For this reason, the multicolor ink-jet system is now being widely adapted even to the field of full-color image recording, because this system affords a recorded image at a lower cost compared with the photographic process in the use field where the required number of copies is small enough.
In the ink-jet recording system, efforts have been made on the part of equipment and ink composition so that coated and non-coated paper commonly used for general printing or writing purposes may be used as the recording medium. However, with the improvement or refinement in the performance of equipment and the enlargement in use field, such as increase in running speed of equipment, refinement of equipment, and full-color recording, it has become necessary for the recording medium to have higher characteristics such as higher density and brighter or more brilliant tone of ink dots; improved ink absorbency to prevent the ink dots from running or feathering even when ink dots have been overlapped; more controlled lateral diffusion of ink dots so that each ink dot may have smooth and not blurred contour. It is further required that when exposed to ultraviolet light, atmospheric oxygen or moisture, the fastness of dyes in the ink is not deteriorated but preferably is increased.
To answer the above requirements, several proposals have been made. For instance, Japanese Patent Application "Kokai" (Laid-open) Nos. 53,012/77 and 49,113/78 have disclosed respectively an ink-jet recording paper comprising a low-sized base paper impregnated with a surface coating composition and that comprising a paper sheet containing an internally added urea-formaldehyde resin powder and impregnated with a water-soluble polymer. These ink-jet recording paper sheets of the plain paper type rapidly absorb an ink, but have disadvantages of blurred dot contour and low dot density.
Japanese Patent Application "Kokai" (Laid-open) Nos. 5,830/80, 51,583/80, and 11,829/80 have disclosed respectively an ink-jet recording paper sheet provided with an ink absorptive coating layer on a base substrate; examples of said coating layer containing a non-colloidal silica powder as pigment; and examples of coated paper carrying a coating of two-layer structure in which the ink absorption speed is different from each other. These inkjet recording paper sheets of the coated type are improved in dot diameter, dot shape, dot density, and color tone reproducibility as compared with those of the plain paper type, but the inks used for those recording sheets are in most cases water-based inks employing a water-soluble dye and, as a consequence, when the image formed on the recording medium is exposed to water, the dye tends to be leached out of the image, resulting in marked decrease of the record value. To overcome these difficulties, there have been disclosed in Japanese Patent Application "Kokai" (Laid-open) No. 53,591/80 examples of applying a water-soluble metal salt to the recording surface; in No. 84,992/81 examples of recording media containing a polycationic polyelectrolyte in the surface; in No. 150,396/80 a method of applying onto the ink-jet record a water-proofing agent capable of forming a lake with the dye in ink dots; and in No. 58,869/81 a method which comprises forming an ink-jet record on a recording medium carrying a coating layer of a water-soluble polymer and then insolubilizing the water-soluble polymer to effect water proofing treatment. These water-proofing methods, however, have a disadvantage of either insufficient waterproofing effect or decreased durability of the dye owing to some reaction between the water-proofing agent and the dye. As a consequence, it has been quite difficult to establish sufficient water-proofness and light fastness at the same time.
For the purpose of improving the light fastness, there have been disclosed in Japanese Patent Application "Kokai" (Laid-open) Nos. 68,303/79, 85,804/79, and 18,151/81 examples in which ultraviolet absorbers are incorporated into the ink. These UV absorbers, however, present problems such as decreased stability of the ink jet and insufficient effect of the addition in small amounts. As an alternative solution of the problem, it has been proposed to incorporate UV absorbers such as a benzophenone type or a benzotriazole type into the ink-jet recording sheet, as disclosed in Japanese Patent Application "Kokai" (Laid-open) Nos. 74,192/82, 74,193/82, and 87,988/82. Such a method, however, has disadvantages of insufficient effect on the light fastness and low efficiency of UV absorbers owing to limited solubility of UV absorbers in water.